Variable wavelength light source

ABSTRACT

When a piezoelectric element  3  applies an external force to a plastic photonic crystal, the photonic crystal deforms, and accordingly, the photonic band gap easily changes. When the photonic band gap changes, transmission of light with a specific wavelength is limited. Therefore, light with a desired wavelength is outputted from the photonic crystal  2  upon sufficient tuning, and extracted to the outside through an output window  6 . In the present invention, a plastic photonic crystal  2  which can achieve sufficient wavelength tuning although it is small is used, and elements are unitized, so that the entire wavelength tunable light source unit is downsized.

TECHNICAL FIELD

This invention relates to a wavelength tunable light source unit.

BACKGROUND ART

A semiconductor single crystal is a material composed of a periodic andregular configuration of specific atoms. Its electron propagationproperty is determined by the interatomic spacing in the semiconductorcrystal. Namely, a semiconductor has an energy band gap, and the energyband gap is determined depending on the wave nature of electrons and theperiodic potential of atoms.

On the other hand, a photonic crystal is a material having opticalpotential differences, that is, a three-dimensional structure composedof substances with refractive index differences arranged in periodsequivalent to light wavelengths. Such a photonic crystal was suggestedby Yablonovich and others.

In a photonic crystal, the optical propagation property is limited by abinding condition concerning the wave nature of light. Namely, opticalpropagation in a photonic crystal is limited as well as electronpropagation in a semiconductor. In a photonic crystal, an opticalforbidden band, that is, a photonic band gap exists, and due to thisexistence, light in a specific waveband cannot propagate in the crystal.

Conventionally, various photonic crystals have been suggested. Forexample, a photonic crystal composed of submicron particles arranged inperiods equivalent to light wavelengths exists. For microwaves, aphotonic crystal composed of polymer particles arranged in a space isgenerally known.

In addition to these, there are various photonic crystals such as aphotonic crystal that has periodic minute spaces formed in a metal bychemically dissolving polymer particles after solidifying the polymerparticles in the metal, a photonic crystal with pores perforating atequal intervals into a metal, a photonic crystal which includes regionsthat are formed in a solid material by a laser so as to be different inrefractive index from other regions, and a photonic crystal ofphoto-induced polymers formed in a groove shape by means of lithography.A photonic crystal formed by the abovementioned processing has aphotonic band gap uniquely determined by the structure.

A wavelength tunable light source unit using such a photonic crystal canselect and output a predetermined wavelength range of input light. Inthe description given below, light to be inputted into the photoniccrystal is regarded as input light, and light to be outputted from thephotonic crystal after being transmitted through the photonic crystal isregarded as output light.

DISCLOSURE OF INVENTION

However, in the wavelength tunable light source unit, the wavelength ofoutput light thereof cannot be tuned since the photonic band gap of thephotonic crystal cannot be sufficiently changed. he invention has beendeveloped in view of these circumstances, and an object thereof is toprovide a wavelength tunable light source unit which can sufficientlytune the wavelength of output light by employing a photonic crystal thatis deformable by an external force.

A wavelength tunable light source unit relating to the inventioncomprises a plastic photonic crystal, external force applying means forapplying an external force to the photonic crystal, a light source forinputting light with a plurality of wavelengths into the photoniccrystal, and an output part for outputting light in a predeterminedwaveband selected by the photonic crystal, which are installed in aunit.

Light inputted into the photonic crystal from the light source changesits waveband in accordance with an external force applied by theexternal force applying means. Namely, the waveband of output lightchanges in accordance with the photonic band gap of the photoniccrystal. In the invention, since the photonic crystal is plastic, thewaveband of output light can be greatly changed, and since the elementsare installed in a unit, the entire light source unit becomes compact.

In a case where the light source is a lamp, light outputted from thelamp is led into the plastic photonic crystal, a predetermined wavebandis selected in accordance with the photonic band gap of the photoniccrystal, and then light in the selected waveband is outputted to theoutside via the output part.

In a case where the light source is a laser light source, lightoutputted from the laser light source is led into the plastic photoniccrystal, a predetermined waveband is selected in accordance with thephotonic band gap of the photonic crystal, and then light in theselected waveband is outputted to the outside via the output part.

Particularly, in the present invention, the abovementioned laser lightsource is a semiconductor laser having two opposed end faces foremitting light, light outputted from one of the end faces is inputted toa reflecting mirror through the photonic crystal, a laser resonator isconstructed between the reflecting mirror and the other end face of thesemiconductor laser, the other end face is optically coupled with theoutput part, and the waveband of light outputted from the output partchanges in accordance with an external force that is applied by theexternal force applying means.

In this case, the photonic crystal is disposed inside the laserresonator between the reflecting mirror and the other end face, so thata resonant wavelength is selected by the photonic crystal, and light inthe selected waveband is outputted to the outside via the output part.

It is also possible that the laser light source is a titanium sapphirelaser including a resonator that contains a photonic crystal, and thetitanium sapphire laser can generate pulsed light at intervals offemtoseconds, and the waveband thereof can be changed by the photoniccrystal disposed inside the resonator.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view of a wavelength tunable light source unit.

FIG. 2 is a perspective view of a photonic crystal 2.

FIG. 3 is a graph showing wavelength (nm) dependence of thetransmittance (optional constant) of output light through a dichroicmirror.

FIG. 4 is an explanatory view of a wavelength tunable light source unitrelating to another embodiment.

FIG. 5 is an explanatory view of a wavelength tunable light source unitrelating to still another embodiment.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, a wavelength tunable light source unit of an embodiment isdescribed. The same symbol is used for the same elements or elementshaving the same function, and overlapping description is omitted.

FIG. 1 is an explanatory view of a wavelength tunable light source unit.This wavelength tunable light source has a light source 5 for emittinglight with a plurality of wavelengths on a base, and light outputtedfrom the light source 5 is inputted into a photonic crystal 2. Thephotonic crystal 2 is placed on the base 1. The photonic crystal 2 isenergized by a piezoelectric element (external force applying means) 3which applies a pressure on the crystal and reduces the pressure appliedon the crystal. The photonic crystal 2 selects a desired waveband fromthe waveband of input light and outputs light in the selected wavebandas output light through an output window (output part) 6. These elements2, 3, and 5 are disposed in a cover member C that forms a housing inconjunction with the base 1 and the output window 6, and are unitized.

The photonic crystal 2 is a material which accurately deforms by beingapplied with an external force and changes the photonic band gap inaccordance with the deformation. When its photonic crystal 2 is deformedby the piezoelectric element 3, the photonic band gap changes. Thestrength of the external force and the period of applying the force bythe piezoelectric element 3 are controlled by a driving device 4.

Input light outputted from the light source 5 is inputted into thephotonic crystal 2, and specific wavelength components in the inputlight cannot pass through the photonic crystal 2, and a predeterminedwaveband is selected in accordance with the photonic band gap (opticalresponse characteristic) and outputted from the photonic crystal 2 asoutput light. The output light is inputted into the output window 6 thatpropagates light, and outputted toward the outside of this wavelengthtunable light source unit through the output window 6. Namely, theoptical coupling performance between the light source 5 and the outputwindow 6 changes due to application of an external force.

Although this wavelength tunable light source unit changes the photonicband gap of the photonic crystal 2 by applying an external force to thephotonic crystal 2, the photonic crystal 2 is plastic. Furthermore, thephotonic crystal 2 may have elasticity.

Since the photonic crystal 2 is plastic, when an external force isapplied to this crystal to deform it, the photonic band gap greatlychanges and the wavelength of output light from the photonic crystal 2sufficiently changes. In such a wavelength tunable light source unit,even when the volume of the photonic crystal 2 is reduced, wavelengthselection can be effectively carried out, so that the entire lightsource unit can be downsized.

As described above, the wavelength tunable light source of the presentembodiment comprises the plastic photonic crystal 2, the piezoelectricelement 3 for applying an external force to the photonic crystal 2, thelight source 5 for inputting light with a plurality of wavelengths intothe photonic crystal 2, and the output window 6 for outputting light ina predetermined wavelength band that is selected by the photonic crystal2, which are installed in a unit 2.

The light inputted into the photonic crystal 2 from the light source 5changes its waveband in accordance with an external force applied by thepiezoelectric element 3. Namely, the waveband of the output lightchanges in accordance with the photonic band gap of the photonic crystal2. Since the photonic crystal 2 is plastic, they can greatly change thewaveband of the output light, and since these elements are installed ina unit, the entire light source unit becomes compact.

In a case where the light source 5 is a lamp such as a mercury lamp,light outputted from the lamp is led into the plastic photonic crystal2, and light in a predetermined waveband is selected in accordance withthe photonic band gap of the photonic crystal 2 and outputted to theoutside through the output window 6.

In this example, the photonic crystal 2 is a gel material, and is housedin a transparent container V.

In the case where the light source 5 is a laser light source, lightoutputted from the laser light source is led into the plastic photoniccrystal 2, and light of a predetermined waveband is selected inaccordance with the photonic band gap of the photonic crystal 2 andoutputted to the outside through the output window 6.

FIG. 2 is a perspective view of the photonic crystal 2.

The photonic crystal 2 contains a plurality of microspheres (opticalmicrocrystals) of silica or barium titanate in a gel material 2G. Thisphotonic crystal 2 can be easily deformed. The microspheres 2B areregularly and evenly arranged in the material 2G in periods equivalentto light wavelengths. The interval of the microspheres 2G is between aquarter and a half of the light wavelength to be selected, and themicrospheres 2B have permeability with respect to this wavelength. Whenlight in a waveband Δλ (including λ₁) enters the photonic crystal 2,only components in a specific waveband λ₁ in accordance with thephotonic band gap are transmitted through the photonic crystal 2.

Since the gel is easily deformed by an external force, the photonic bandgap of the photonic crystal 2 easily changes. In accordance with thischange, the abovementioned waveband λ₁ passing through the photoniccrystal 2 changes. The microspheres 2B and the material 2G are differentin reflective index from each other, and both of these have permeabilitywith respect to the selected light wavelength.

For example, a sol material mixed with an ultraviolet curing resin isused, and is gelled by being exposed to ultraviolet radiation. A typicalultraviolet curing resin contains a cross-linking agent and a photopolymerization initiator which are mixed with acrylamide, and variousultraviolet curing resins of this type have been generally known.

The number of periodic structures of the microspheres 2B is about 50, sothat the photonic crystal 2 can sufficiently function even with a 100 μmsquire size at the maximum. Therefore, use of this photonic crystal 2can achieve downsizing of the unit. It is also possible that bubbles areused in place of the microspheres 2B.

FIG. 3 is a graph showing wavelength (nm) dependence of thetransmittance (optional constant) of output light through a multilayerphotonic crystal, that is, a dichroic mirror. The input light is whitelight. The characteristic shown in this graph is not of theabovementioned photonic crystal 2, however, in a case where themicrospheres 2B are arranged at completely equal intervals, the opticalcharacteristic of the photonic crystal 2 becomes identical to that shownin this figure with respect to a specific direction. In this example,the transmittance of light around a 400 nm waveband is lower than thetransmittances of other wavebands.

FIG. 4 is an explanatory view of a wavelength tunable light sourcerelating to another embodiment. This wavelength tunable light sourceuses a semiconductor laser having two opposed end faces 5A and 5B foremitting light as the abovementioned laser light source 5. Lightoutputted from one end face 5A is inputted to a reflecting mirror 7through a photonic crystal 2, and a laser resonator is constructedbetween the reflecting mirror 7 and the other end face 5B. The photonicband gap in such a photonic crystal 2 is set upon consideration of abasic wave and a harmonic wave of the laser light source so thatresonance is performed at these wavelengths.

The core of an optical fiber is inserted into an opening made in thecover member C, and the front end of the core 6 is placed on a V-groovedbase 1V fixed on the base 1, and the semiconductor laser 5 is fixed on aheat sink 1H that is fixed on the base 1.

The other end face 5B of the semiconductor laser 5 is optically coupledwith the core 6 that serves as an output part 6, and the waveband oflight outputted from the core 6 changes in accordance with an externalforce applied by the piezoelectric element 3.

The core is surrounded by a clad 6′, and these form an optical fiber.

In the present embodiment, the photonic crystal 2 is disposed in a laserresonator constructed between the reflecting mirror 7 and the other endface 5B, so that a resonant wavelength is selected by the photoniccrystal 2, and light in the selected wave band is outputted to theoutside through the output part 6.

Furthermore, a commercially available wavelength tunable light sourcewhich rotates a diffraction grating and makes it to serve as awavelength selector to select and output a specific wavelength has beengenerally known. Although such a commercially available light source islarge-scale, the wavelength tunable light source unit of theabovementioned embodiment can be made compact in comparison with thecommercially available light source since the plastic photonic crystal2, that is, a gelled photonic crystal is used.

For example, it is also possible that the photonic crystal 2 is producedby using the semiconductor micro-machining technology (microelectro-mechanical systems: MEMS technology). The abovementionedcontainer V is formed by processing a semiconductor substrate (notshown), and a piezoelectric element 3 is formed on this semiconductorsubstrate. In this case, the photonic crystal 2 is disposed inside thecontainer formed of the semiconductor substrate, more specifically,inside a concave portion, and the piezoelectric element 3 is formed onthis semiconductor substrate, so that these elements can be formed byusing the semiconductor micro-machining technology, and the entire unitcan be downsized. As a matter of course, it is also possible that adrive circuit, a power source, and a photodiode with a wavelengthfilter, etc., can be formed inside the semiconductor substrate.

FIG. 5 is an explanatory view of a wavelength tunable light sourcerelating to still another embodiment. This wavelength tunable lightsource is different from the light source shown in FIG. 4 in that anexcitation light source 5E and a laser medium 5M are used for the laserlight source 5, and a laser resonator including the laser medium 5M isconstructed between an output mirror 6 that is provided in place of theoptical fiber core 6 and a reflecting mirror 7.

Excitation light emitted from the excitation light source 5E is inputtedinto the laser medium 5M, and the laser medium 5M is excited to output alaser beam from its end face. The emitted laser beam is amplified andresonated while reciprocating between the two reflecting mirrors 6 and 7that are disposed opposite to each other in the unit, and the amplifiedlaser beam is outputted to the outside through the reflecting mirror(output mirror) 6 having the lower reflectance.

A laser beam wavelength which can reciprocate in this laser resonator islimited by the photonic crystal 2, and is changed by an external forceapplied by the piezoelectric element 3. In this example, the waveband ofthe output light also changes in accordance with the amount of drivingof the piezoelectric element 3.

In this example, the laser medium 5M is Ti sapphire. The laser lightsource 5 using this is a titanium sapphire laser including a resonatorthat contains the photonic crystal 2. The titanium sapphire laserchanges the time interval as well as the wavelength. The titaniumsapphire laser can generate pulsed light at femtosecond time intervals,and the waveband thereof can be changed by the photonic crystal 2disposed inside the resonator. Furthermore, it is also possible that aCr³⁺:LiSrAlF₆ crystal or a Cr³⁺:LiCaAlF₆ crystal is used as the lasermedium 5M.

A Fabry-Perot interferometer and a multilayer mirror (dichroic mirror)also use zero-dimensional or one-dimensional photonic crystals. Thephotonic crystal 2 can also be used for these purposes. Furthermore, itis expected that, regarding the soft photonic crystal 2 mentioned above,research will be further advanced in future on the size or arrangingstability of the microspheres 2B or bubbles, mechanical accuracy toimprove controllability, long-term stability of the gel, temperaturestability, method of connection to the optical fiber or other opticalparts, the gel sealing container, and the external force applyingmechanism which can apply an equal external force each time.

INDUSTRIAL APPLICABILITY

The present invention can be used for a wavelength tunable light sourceunit.

1. A wavelength tunable light source unit, comprising: a plasticphotonic crystal; external force applying means for applying an externalforce to said photonic crystal; a laser light source for inputting lightwith a plurality of wavelengths into said photonic crystal; and anoutput structure for outputting light in a predetermined waveband thatis selected by said photonic crystal; wherein the laser light source isa semiconductor laser having two opposed end faces for emitting light,light outputted from one of the end faces is inputted to a reflectingmirror through said photonic crystal, a laser resonator is constructedbetween said reflecting minor and the other one of the end faces, andthe other one of the end faces is optically coupled with said outputstructure, and the waveband of light to be outputted from said outputstructure changes in accordance with an external force applied by saidexternal force applying means.
 2. The wavelength tunable light sourceunit according to claim 1, wherein said output structure comprises anoutput window.